The present invention relates generally to apparatuses for reproducing a video signal of a certain system recorded on a rotary recording medium as a video signal of a different system. More specifically, the invention relates to an apparatus for reproducing a video signal recorded on a rotary recording medium as a video signal of one out of a plurality of different systems which are classified according to difference in the field frequency and the number of horizontal scanning lines as a video signal substantially of another system.
At present, there are two principal video signal systems for television. One system, used in the United States, Japan, and other countries (hereinafter referred to as the "first standard system" or the "first system") is based on a field frequency of 60 Hz (precisely speaking, 59.94 Hz in the case of a color video signal) with a number of horizontal scanning lines of 262.5 lines within one field. The other system, used in Europe and other areas (hereinafter referred to as the "second standard system" or the "second system") is based on a field frequency of 50 Hz with 312.5 horizontal scanning lines within one field. There are various other systems such as those of combinations of field frequencies and numbers of horizontal scanning lines within one field of 50 Hz and 202.5 lines and 50 Hz and 409.5 lines. However, with the full-scale spread of color broadcasting, the trend of intensification is toward the above mentioned first and second standard systems.
As is known, video signal systems can be further classified by the mode of transmission of the chrominance signal as the NTSC system, the PAL system, and the SECAM system in addition to the above described classification based on differences in field frequency and number of horizontal scanning lines within one field. With respect to this difference in mode of transmission of the chrominance signal, mutual conversion can be carried out with relative ease by an electrical process.
However, so-called system conversion, wherein field frequency and number of horizontal scanning lines within one field are converted, has heretofore required very complicated and elaborate apparatus. More specifically, the necessity for so-called system conversion, wherein field frequency and number of horizontal scanning lines are converted, has heretofore been primarily a problem mutually between broadcasting stations or networks as in international relay broadcasting between different regions (for example, between Europe and the United States). For this reason, it has been necessary to carry out this system conversion in a manner to meet very strict standards. Consequently, elaborate and large-scale electronic conversion apparatuses have been employed.
In an advanced electronic conversion apparatus being used at present, a system by which a video signal is once converted into a digital signal, which is then stored in a digital memory of large capacity, subsequently read out in a required sequence, and restored into the original video signal is used. By manipulating the time ratio of this signal storing and reading out, conversion of field frequency and number of horizontal scanning lines is accomplished. This electronic conversion apparatus, however, is disadvantageously elaborate and expensive.
On the other hand, a reproducing apparatus has been reduced to practice, which is adapted to reproduce a video signal recorded on a rotary recording medium (so-called video disc, and hereinafter simply referred to as a "disc") in a reproducing system such as an electrostatic or an optical reproducing system. The disc reproducing apparatus only reproduces a standard video signal which is recorded on the disc. Accordingly, it is not necessary for the modulation format of a chrominance signal to conform to the NTSC system, the PAL system, or the SECAM system. It is sufficient to convert the chrominance signal into a predetermined modulation format which conforms to a television system of a television receiver, when the disc reproducing apparatus produces and supplies the standard color video signal to the above television receiver. Hence, by incorporating a conversion device for obtaining a specific carrier chrominance signal of the NTSC system, the PAL system, or the SECAM system, in the disc reproducing apparatus, the disc reproducing apparatus can be made compatible with respect to another reproducing apparatus having a different chrominance signal transmission format.
Therefore, compatibility of the disc reproducing apparatus can be obtained by considering the difference in the chrominance signal transmission format described above. Since the difference between the horizontal scanning frequencies in differing television systems is 0.7% which indicates that the horizontal scanning frequencies are approximately equal, the horizontal scanning frequency within the disc reproducing signal can be reproduced at an original horizontal scanning frequency by the disc reproducing apparatus, by controlling the rotational speed of the disc. Thus, the rotational speed of the disc was conventionally controlled by performing a servo operation so that the frequencies of the reference signal in a turntable servo circuit and the reproduced horizontal synchronizing signal, respectively coincide. However, in this conventional apparatus, a predetermined servo operation could not be performed when the rotational speed of the disc was not close to the regular rotational speed.